Experimental verification of laser-induced fluorescence based on the velocity distribution of helium ash in multi-dipole device*

Future fusion devices, such as ITER, based on the fusion of deuterium and tritium (D-T), will have to deal with helium as the 'ash' byproduct. Continuous operation is not possible at the maximum helium content in the plasma, where significant dilution of the D-T fuel and radiation of plasma thermal energy occur. Where helium ash is produced by the D-T process, the confinement period of helium ash is dominated by edge transport, recycling, and edge pumping rate rather than core transport. Consequently, the discharge of helium ash in the edge region, particularly the discharge of helium neutral particles in the divertor region, is a crucial factor determining the future stability of nuclear reaction processes. The non-invasive diagnostic method of laser-induced fluorescence (LIF) is expected to be used extensively to study the dynamics of helium metastable neutral particles via direct measurement of the neutral particles velocity distribution function (NVDFs) in order to monitor helium ash exhaust in fusion devices. A multi-dipole chamber in this work was employed to simulate the existence of helium neutral particles near the divertor of a fusion reactor. In addition, the laser wavelength calibration, data analysis automation,  and the required tunable range of a laser to cover the NVDF and efforts to extend the tunable range to meet this requirement were discussed in detail. These works will enable us in developing a feasible LIF diagnosis technology scheme suitable for the divertor region of the future fusion reactor, determining the velocity distribution of helium neutral particles near the divertor, and monitoring helium ash exhaust.